Steam temperature control system

ABSTRACT

A steam temperature control system for a double reheating, forced circulation steam-generating unit is provided. The heat recover area of said unit is divided into two gas passages, a high-pressure reheater being disposed in one of the passages while a low-pressure reheater, in the other passage. A surfacetype heat exchanger of the shell and the tube type is interposed in said low-pressure reheater so that the heat is transferred from the superheated steam bypassed from the main steam passage of said unit to the steam in said low-pressure reheater. By varying the temperature difference between said two steams, the quantity of heat to be transferred may be adjusted, thereby controlling the temperature of the steam in said low pressure reheater.

United States Patent Soh 1 1 Feb. 22, 1972 [54] STEAM TEMPERATURE CONTROL 3,345,975 10/1967 Stevens et al. ..l22/479 X SYSTEM 3,368,534 2/1968 Gorzegno et 31.. ..l22/480 3,472,209 10/1969 Roffler ..122/483 X [72] Inventor: Dohd Soh, Yokohama-shi, Japan [731 Assignee: lshikawajlma-l-larima Jukogyo Kabushlld WWW-Kenna Kaisha, Tokyoto, Japan Attorney-None and Noite [22] Filed: Dec. 8, 1969 [57] ABSTRACT {21] Appl. No.: 882,993 A steam temperature control system for a double reheating, forced circulation steam-generating unit is provided. The heat recover area of said unit is divided into two gas passages, a [30] Fords Application Priority high-pressure reheater being disposed in one of the passages June 1 l, 1969 Japan ..44/45452 While a 'p in the Other P 8 A Surfacetype heat exchanger of the shell and the tube type is inter- 52 us. C1 ..122/483 P in said low-Pressure Wheat" 50 that the heat is trans- [511 int. 5/16 fem! from superheated steam bypassed from the 53 Field of Search ..122/478 479 s 480 483 5mm Ofsaid the steam in said heater. By varying the temperature difference between said [56] References Cited two steams, the quantity of heat to be transferred may be adjusted, thereby controlling the temperature of the steam in UN E S ES PATENTS said 10w premtre reheater. 2,966,896 1/1961 Vogler ..l22/479 X 3 Claims, 2 Drawing Flgum r 1 T H T a 1 1 1% t l STEAM TEMPERATURE CONTROL SYSTEM DETAILED EXPLANATION OF THE INVENTION The present invention relates to a steam temperature control system, and more particularly a steam temperature control system for controlling the low-pressure reheated steam temperature, in a double reheating, once-through-type steamgenerating unit.

In regulating the low-pressure reheated steam temperature in the conventional double reheating, once-through-type steam-generating unit, the gas recirculation fans, tilting burner, spray water attemperators etc., are usually employed. However, in case of the gas recirculation fans and the spray water attemperator methods, the power for auxiliary equipment must be increased or the plant efficiency is reduced so that the economy of the plant operation is adversely affected. In case of the burner tilting method, the control of the lowpressure reheated steam temperature is not effectively carried out when only the burner tilting method is employed. Furthermore, as in the case of the gas recirculating fans, the burner tilting adversely affects the high-pressure reheated steam temperature as well as the superheated steam temperature as a disturbance so that there is a problem concerning controllability. There has been also proposed a method in which the heat recovery area of the steam generating unit is divided into two gas passages and a high-pressure reheater is disposed in one of said two passages while a low-pressure reheater is disposed in the other passage and provided with a shell-andtube-type surface-type heat exchanger connected to the heat transfer surface of the low-pressure reheater, whereby the heat is transferred from the low-pressure reheated steam to the feed water which bypassed the economizer, thereby controlling the temperature of the low-pressure reheated steam. This method can eliminate the defects described above to some extent. But there is a problem that the heating surface areas of the low-pressure reheater as well as the surface-type heat exchanger of the shell and tube type cannot be reduced satisfactorily. That is, the principle of this method is such that the heat is transferred from the low-pressure reheated steam to the feed water that is bypassed through the economizer. In other words, a relatively large quantity of heat is transferred to the feed water from the low-pressure reheater which absorbs much heat under the heavy load while a relatively small quan tity of heat is transferred to the feed water from the low-pressure reheater that absorbs a relatively small quantity of heat under a light load, thereby controlling the temperature of the low-pressure reheated steam. Therefore, the low-pressure reheater must have a relatively large heating surface area in order to increase the quantity of heat absorbed thereto. Furthermore, in view of the controllability of the surface-type heat exchanger of the shell and tube type, it must also have a relatively large heat transfer surface area.

Accordingly, one of the objects of the present invention is to provide a steam temperature control system which can reduce the heating surface areas of a low-pressure reheater as well as a surface-type heat exchanger by making full use of the characteristics of once-through-type steam-generating unit.

Another object of the present invention is to provide a steam temperature control system which can control the temperature with a higher degree of accuracy in a simple manner.

In brief, the present invention provides a steam temperature control system for a double reheating, once-through-type steam-generating unit characterized in that the heat recovery area of said steam-generating unit is divided into front and back gas passages; a high-pressure reheater is disposed in one of said two gas passages while a portion or the whole of a low pressure reheater is disposed in the other gas passage; and a surface-type heat exchanger of the shell and tube type is inter posed in said low-pressure reheater, on a portion thereof which is outside the furnace so that the heat is transferred from the superheated steam bypassed from the main steam passage of said steam-generating unit to the steam in said low pressure reheater, whereby by varying the temperature difference between said superheated steam and the steam in said low-pressure reheater, the quantity of heat to be transferred to said steam in said low-pressure reheater may be adjusted, thereby controlling the temperature of the steam reheated in said low-pressure re heater.

The above and other objects, features and advantages of the present invention will become more apparent from the following description of one illustrative embodiment thereof with reference to the accompanying drawings.

FIG. I is a flow diagram for explanation of the principle of the present invention; and

FIG. 2 is a diagrammatic sectional view of one embodiment thereof.

The feed water is fed into a furnace passes 1 from a feed water pump through an economizer. From the furnace passes I, the steam is directed toward a high-pressure turbine 8 through a furnace outlet pressure breakdown control valve 2 connected in parallel with a furnace pass outlet valve 3, a pri mary superheater 4, a superheater isolating valve 5, a platentype superheater 6 and a finishing superheater 7 The steam that has expanded in the high-pressure turbine 8 is made to flow through a high-pressure reheater 9 into an intermediate pressure turbine 10 in which the steam expanded again. From the intermediate pressure turbine 10, the steam is directed into the horizontal-type low-pressure reheater 11 and then into a low-pressure turbine 14 through a surface-type heat exchanger 12 and a pendant-type low-pressure reheater l3. From the low-pressure turbine 14, the steam is directed into a condenser 15.

FIG. 2 is a diagrammatic sectional view of a heat-generating unit embodying the present invention. The fuel is burned by burners I7 in a furnace [8. The combustion gases flow through the platen-type superheater 6, the finishing super heater 7 and the pendant-type low-pressure reheater l3 and thereafter one portion of the combustion gases flows into the front gas passage into which is disposed the high-pressure reheater 9 and after passing through the economizer Ia and a gas damper 190, the combustion gases are mixed with those that have passed through the back gas passage. Thereafter, the gases are finally discharged from the chimney through an air preheater (not shown). In the back gas passage are disposed the horizontal-type low-pressure reheater, the primary superheater 4, the economizer lb and a gas damper [9b in the order named. The shell-and-tubeaype heat exchanger 12 is installed on a portion of the outlet of the horizontal-type low-pressure reheater I], said portion is located outside the furnace, and the reheated system by the surface-type heat exchanger 12 of the shell and tube type is directed into the pendant-type lowpressure reheater I3. In this case, the control of the temperature of the steam heated through the high-pressure reheater is made by adjusting the degree of opening of the dampers 19a and 191: which are so arranged that one of them opens while the other closes and vice versa based upon the well-known gas bias control system.

On the other hand, the temperature of the steam heated through the low-pressure reheater is controlled based upon the control system in accordance with the present invention. That is, the heat to be transferred from the high-pressure, su perheated steam to the low-pressure reheated steam is con trolled. More particularly, the high-pressure steam flowing through the platen-type superheater 6 is bypassed into the surface-type heat exchanger I2 of the shell and tube type where the high-pressure steam is subjected to the heat exchange with the reheated steam. Thereafter, the bypassed high-pressure steam is returned to the outlet of the platen-type superheater 6. The quantity of heat to be transferred may be controlled by adjusting the flow rate of the superheated steam flowing through the heat exchanger I2 by operating the control valve l6 disposed at the inlet of the surface-type heat exchanger 12 so that the average temperature difference between the superheated steam and the reheated steam may be suitably varied. It is understood that instead of the control valve 16, a waterinjection-type temperature attemperator device or the like may be employed. As the heat exchanger 12, the shell-andtube type is employed. In the shell-and-tube-type heat exchanger, the superheated steam is made to flow through the tube while the low-pressure reheated steam is made to flow through the shell.

A known type of heat exchanger is the well-known double pipe arrangement which is installed in the path of the combustion gases of a furnace. in this arrangement the superheated high-pressure steam is created in the inner pipe of the double pipe, and the lowpressure double reheated steam is caused to flow in a circular path between the inner and outer pipes. However, the present heat exchanger can be manufactured more easily than the double-pipe-type heat exchanger. Another advantage of the present construction is that it can be repaired or inspected with ease since it is located outside the furnace, and if there is a leakage on the high-pressure pipe in the shell the leaking high-pressure pipe can be easily plugged, ln the instant embodiment, the surface-type heat exchanger of the shell and tube type is interposed in the low-pressure reheater, but it may be disposed at the inlet or the outlet of the low-pressure reheater.

As described hereinabove, according to the present invention the heat is transferred from the superheated steam to the low-pressure reheated steam in the once-through-type steam generating unit so that when the heat absorption of low-pressure reheater is not sufficient at light load, the quantity of heat to be transferred from the superheated steam to the low-pressure reheated steam may be increased accordingly. For example, as shown in FIG. I by opening the control valve 16, the low-pressure reheated steam may be increased in temperature. The temperature of the superheated steam may be freely varied by varying the ratio between the feed water flow and the fuel flow because of the inherent characteristic of the once-through-type steam-generating unit so that the control range of the temperature of the superheated steam is entirely independent of the arrangement of the heat transfer surface area of the superheater. Furthermore, according to the present invention, the quantity of heat required for controlling the temperature of the steam heated through the low-pressure reheater, that is, control duty is obtained from the superheater through the surface-type heat exchanger of the shell and tube type so that the superheated steam temperature characteristic is not adversely affected at all. Therefore, not only the heating surface area of the low-pressure reheater may be reduced, but also the heating surface of the heat exchanger may be reduced as the superheater intermediate steam temperature has a remarkable radiation characteristic under light load in the once through-type steam-generating unit which facilitate the heat exchanger design. This can be explained in the following way, the platen-type superheater 6 in FIG. t has such a remarkable radiation characteristic that even when the control valve 16 is left opened at a fixed position the temperature difference between the superheated steam and the low-pressure reheated steam is increased under the light load so that the quantity of heat to be transferred from the superheated steam to the lowpressure reheated steam is increased therefore the heating sur face of heat exchanger can be reduced. This is a very desirable characteristic of the steam temperature control in the low pressure reheater, Furthennore, the steam temperature control system in accordance with the present invention can eliminate the increase of the power for auxiliary equipment and the decrease in the efficiency of the steam generating unit Moreover, the steam temperature control system of the present invention will not adversely affect the high-pressure reheated steam temperature as well as the superheated steam temperature.

The present invention has been so far described with particular reference to one illustrative embodiment thereof, but it is understood that variations and modifications can be effected without departing from the true spirit of the present invention as described hereinabove and as defined in the appended claim.

What is claimed is:

l. A steam temperature control system for a double reheating, once-through steam-generating apparatus comprising a furnace for generating steam, at least one superheater for said steam, a heat recovery unit divided into front and back gas passages, a high-pressure reheater located in one of said gas passages, a low-pressure reheater located in the other of said gas passages, a shell-and-tube-type heat exchanger secured to a portion of said heat recovery unit outside of said furnace, a portion of said lowpressure reheater being disposed in said shell-and-tube-type heat exchanger, and valve means in said control system, whereby the superheated steam is bypassed to said shell-and-tube-type heat exchanger in heat exchange relationship with said portion of the reheater located therein whereby by varying the temperature difference between said superheated steam and the steam in said low-pressure reheater the quantity of heat to be transferred to said steam in said lowpressure reheater may be adjusted, thereby controlling the temperature of the steam reheated in said low-pressure reheater.

2. A steam temperature control system as claimed in claim I wherein said valve means is a volume adjustment valve in the inlet of said heat exchanger.

3. A steam temperature control system for a double reheating, once-through steam-generating apparatus comprising a furnace for generating steam, at least one superheater for said steam, a heat recovery unit divided into front and back passages, a high-pressure reheater located in one of said passages, a low-pressure reheater having at least a portion thereof disposed in the other of said gas passages. a gas damper positioned at the outlet of the Iowtemperature portion of each of said gas passages, the volume of the combustion gas flow passing through said highpressure reheater being adjustable by opening and closing said gas damper so as to control the steam temperature at the outlet of the high pressure reheater, a shell-and-tube-type heat exchanger secured to a portion of said heat recovery unit outside of said furnace, a portion of said low-pressure reheater being disposed in said shell-and-tubetype heat exchanger, means bypassing steam from the main path of the furnace steam generation and through said shell-and-tube-type heat exchanger whereby the low-pressure reheated steam is provided with heat from said superheated steam that has a higher temperature than the low-pressure reheated steam, and a flow volume adjusting valve fixed to the inlet of said heat exchanger whereby the superheated steam is bypassed to said shell-and-tube-type heat exchanger of the reheater located therein for altering the volume of flow of said bypassed superheated steam so that the quantity of heat to be transferred to said lowpressure reheated steam is changed in said heat exchanger a s a a t 

1. A steam temperature control system for a double reheating, once-through steam-generating apparatus comprising a furnace for generating steam, at least one superheater for said steam, a heat recovery unit divided into front and back gas passages, a highpressure reheater located in one of said gas passages, a lowpressure reheater located in the other of said gas passages, a shell-and-tube-type heat exchanger secured to a portion of said heat recovery unit outside of said furnace, a portion of said low-pressure reheater being disposed in said shell-and-tube-type heat exchanger, and valve means in said control system, whereby the superheated steam is bypassed to said shell-and-tube-type heat exchanger in heat exchange relationship with said portion of the reheater located therein whereby by varying the temperature difference between said superheated steam and the steam in said low-pressure reheater the quantity of heat to be transferred to said steam in said low-pressure reheater may be adjusted, thereby controlling the temperature of the steam reheated in said lowpressure reheater.
 2. A steam temperature control system as claimed in claim 1 wherein said valve means is a volume adjustment valve in the inlet of said heat exchanger.
 3. A steam temperature control system for a double reheating, once-through steam-generating apparatus comprising a furnace for generating steam, at least one superheater for said steam, a heat recovery unit divided into front and back passages, a high-pressure reheater located in one of said passages, a low-pressure reheater having at least a portion thereof disposed in the other of said gas passages, a gas damper positioned at the outlet of the low-temperature portion of each of said gas passages, the volume of the combustion gas flow passing through said high-pressure reheater being adjustable by opening and closing said gas damper so as to control the steam temperature at the outlet of the high-pressure reheater, a shell-and-tube-type heat exchanger secured to a portion of said heat recovery unit outside of said furnace, a portion of said low-pressure reheater being disposed in said shell-and-tube-type heat exchanger, means bypassing steam from the main path of the furnace steam generation and through said shell-and-tube-type heat exchanger whereby the low-pressure reheated steam is provided with heat from said superheated steam that has a higher temperature than the low-pressure reheated steam, and a flow volume adjusting valve fixed to the inlet of said heat exchanger whereby the superheated steam is bypassed to said shell-and-tube-type heat exchanger of the reheater located therein for altering the volume of flow of said bypassed superheated steam so that the quantity of heat to be transferred to said low-pressure reheated steam is changed in said heat exchanger. 